A cross section of a conventional print head for use in a thermal printer is shown in FIG. 4, where a glaze layer 2, a resistor layer 3 as made from a film of Ta.sub.2 N for producing heat, a conductor layer 4 as made from a film of Al for supplying electric power, and a protective layer 5 are stacked on a substrate 1 of alumina in this order. A heater portion 6 produces heat to give thermal energy to thermosensitive paper 7, for making a visible mark.
In the print head shown in FIG. 4, the heater portion 6 is concave in shape and so it fails to make intimate contact with the thermosensitive paper 7, deteriorating the heat transfer efficiency. In an attempt to avoid this difficulty, an improved print head shown in FIG. 1 has been proposed. This head is characterized by the formation of an insulator layer 8 beneath a heater portion 6 and on a glaze layer 2. Thus, the heater portion 6 protrudes so that the head may make intimate contact with thermosensitive paper, thus improving the heat transfer efficiency. Specific examples of this proposed head are next described.
Japanese Patent Laid-Open No. 123442/1975 has disclosed a method of fabricating such a structure by forming an insulator layer of SiO.sub.2 that can be readily etched over the whole surface of a glaze layer as mentioned above and then etching away the layer except for the portion that bears on the heater portion. According to this method, however, the following problem arises during etching process. Since glaze layers usually used for print heads of thermal printers are required to have resistance to heat, they are made from a glass material containing SiO.sub.2 as its main constituent and having a high melting point. This makes it difficult to selectively etch the film of SiO.sub.2 and the underlying glaze layer. Thus, when the film of SiO.sub.2 is etched, the underlying glaze layer is eroded. Then, the surface of the eroded glaze layer becomes very rough. As a result, the formation of a wiring pattern in a conductor layer formed on this glaze layer is adversely affected. Also, it is possible to make the insulator layer from materials other than SiO.sub.2. However, they are limited to insulators of high melting points, because the layer is heated. These insulators of high melting points are generally difficult to etch. In this way, usable etchants are restricted to fluorides, similarly to the case where the insulator layer is made from SiO.sub.2. Hence, problems similar to the foregoing arise. In connection with this, the resistor layer 3 producing heat is formed on the insulator layer 8, as shown in FIG. 1, and a pattern is formed by photoetching. The resistor layer 3 consists mainly of a metal of a high melting point, such as Ta.sub.2 N. Again, etchants of fluorides are usually used. Accordingly, if the insulator layer is made from an etchable material, such as SiO.sub.2, then when the resistor layer is patterned, it is eroded. The result is that the accuracy with which the pattern is formed deteriorates greatly.
Japanese Patent Laid-Open No. 20745/1979 has disclosed another method of forming a film by the use of a metal mask that is obtained by blanking a metal sheet into a desired pattern. More specifically, the metal mask is placed on a glaze layer. An insulator is deposited as a film from above the mask by sputtering or evaporation. Then, the mask is removed to form an insulator layer of a desired pattern on the glaze layer. This method is effective in patterning an insulator layer which is chemically stable and, therefore, difficult to etch. However, the formed pattern varies widely from product to product, resulting in a low manufacturing yield. This is due to the fact that a gap is left between the mask and the glaze layer. This tendency becomes more conspicuous as the size of the substrate increases. Further, while the film is being formed, temperature increase expands the metal mask, enlarging the aformentioned gap. Where the gap exists between the mask and the glaze layer in this manner, particles of the film find entry into the gap during the formation of the film. As a result, the outer portion of the pattern of the insulator layer formed on the glaze layer spreads and becomes extremely unclear. In addition, the shape varies widely from product to product. In this way, when the method of forming a film with a metal mask is utilized, the pattern of insulator layer is fabricated with poor accuracy. Further, the shape of the pattern differs greatly among products. Consequently, this technique is not adapted for the formation of the insulator layer 8 shown in FIG. 1.
In a third method, a photoresist film is formed on a glaze layer except for the portion which makes contact with a heater portion. Then, a chemically stable insulator of a high melting point is sputtered on the photoresist film. Subsequently, the photoresist film is peeled off and removed to leave only the portion of the insulator film which lies under the heater portion as shown in FIG. 1. This method is called lift-off method. Although this method is effective in patterning a film that cannot be etched, it cannot be put into practical use where the temperature of the substrate is elevated as during sputtering process. In particular, even when a magnetron is employed for sputtering, resulting in a relatively small increase of the temperature of the substrate, the temperature reaches as high as 150.degree. to 180.degree. C. during the film growth. Therefore, the previously patterned photoresist is baked. This makes it difficult to lift off the photoresist. This baking cannot be avoided even if a heat-resistant photoresist that begins to flow at a high temperature of 200.degree. C. is used. We assume from this phenomenon that the actual temperature increase on a microscopic scale is considerably high. This temperature might be called the atomic temperature of atoms composing the film and coming from the target during sputtering process.
It is possible to form a film without elevating the substrate temperature by evaporation in vaccuum, in which case the baking of the photoresist can be circumvented and the photoresist can be lifted off. However, when the substrate temperature is low, the formed film does not strongly stick to the underlying layer. Especially, when the head is actually used in a thermal printer, a mechanical force is directly applied to the surface. This will introduce the problem that the film peels off.
Thus, the lift-off method using photoresist poses practical problems associated with the lift-off and removal of the photoresist subsequent to the formation of the film and also with the adhesion of the film. Hence, this method cannot be applied as it is to pattern an insulator layer on a glaze layer beneath the heater portion of the print head of a thermal printer.